digital root

Take a number, n, add its digits,
then add the digits of numbers derived from it, and so on, until the remaining
number has only one digit. This single digit result is called the digital
root of n. For example, in the case of 53281: 5 + 3 + 2 + 8 + 1
= 19; 1 + 9 = 10; 1 + 0 = 1; thus, the digital root of 53281 is 1.